This invention relates to the remediation of contaminated aquifers (groundwater). Chlorinated aliphatics such as tetrachloroethylene or perchloroethylene (PCE), trichloroethylene (TCE), trichloroethane (TCA) and dichloroethylene (DCE) have become common pollutants of soil and groundwater in North America. In the case of TCE this was due to its extensive use as a solvent and degreasing agent in industry, and to spills caused by mishandling, accidental or otherwise. The removal of PCE and TCE from soils and groundwater is an important environmental issue, especially since PCE, TCE and derivatives are potentially toxic to human beings and other life-forms. The invention relates also to other emergent contaminants, such as chloropropanes, namely 1,2,3-trichloropropane (TCP).
Currently, there are several approaches for the biodegradation of chlorinated compounds.
Biodegradation of chlorinated compounds can be accomplished by aerobic microorganisms, namely by methanotrophs or other microorganisms possessing mono-oxygenases with broad selectivity. Aerobic microorganisms (in particular methanotrophs) are capable of efficient mineralization of low chlorinated compounds, while the biodegradation rates of highly chlorinated compounds under aerobic conditions are lower. Moreover, most highly chlorinated chemicals are refractory to conventional aerobic conditions. Polychlorobiphenyls (PCB) such as pentachlorobiphenyl, highly chlorinated monoaromatics such as hexachlorobenzene and 1,2,4,5-tetrachlorobenzene, chlorinated aliphatics such as hexachlorobutadiene, PCE and carbon tetrachloride (CCl4), hetero-substituted aromatics such as 4-chloro-2-nitrophenol (CNP) are not appreciably or not degraded at all under conventional aerobic conditions (Brown et al. 1987; Janssen et al. 1991; Zitomer and Speece 1993; Galli and McCarty 1989; Beunink and Rehm 1990; Field et al. 1995).
Biodegradation of chlorinated compounds can also be accomplished by reductive dechlorination i.e. by anaerobic bacteria e.g. methanogens, which chlorinated compounds. Although the rate of dechlorination of the contaminant itself could be sufficiently high, it decreases with decreasing extent of halogenation (Mohn and Tiedje 1992). Consequently, anaerobic degradation of chlorinated compounds often is incomplete and results in the production of less chlorinated intermediates. These intermediates can be even more toxic than the initial compound. For instance reductive dechlorination of PCE stalls at cis-dichloroethene (cDCE). To date, only one microorganism, Dehalococcoides ethenogenes, has been shown to dechlorinate PCE, TCE or DCE to ethene (Major et al. 2003).
Accordingly, complete biodegradation of these compounds often requires a combination of anaerobic and aerobic conditions. Sequential anaerobic and aerobic biodegradation carried out in two reactors has been demonstrated. It provides complete mineralization of the initial compound. However, the existence of two bioreactor systems (anaerobic and aerobic) increases the cost. As well, a supply of methane is required if the aerobic part is based on methanotrophic activity.
For groundwater applications, reductive dechlorination of PCE or TCE tends to be incomplete while aerobic degradation of TCE occurs in narrow ecological zones due to its specific requirements. In general, anaerobic activity is confined to the centers of contaminant plumes which are usually anaerobic, and aerobic activity occurs at the edges of the plumes where oxygen is present.
Moreover, in view of the low solubility in water of methane (which is required as a carbon source for the methanotrophs), it is difficult to inject enough methane into the system to support sufficient metabolism of the methanotrophs.
An example of a prior art approach is found in U.S. Pat. No. 6,391,184. This patent reports methods for in-situ decontamination of groundwater by producing high amounts of dissolved oxygen and reactive initiators such as hydroxyl radicals. An electrolysis apparatus is described such as to effect the water electrolysis at given depth in screened wells. The apparatus essentially is a probe incorporating a submersible pump, an electrolysis cell, a chlorine filter and distribution chamber. The probe can be introduced into and removed from the well. The incorporation of a pumping device at the probe tip allows for turning the well into a reactive well. The method also includes a protocol to position the wells given the hydrogeology of the site and the extension of its contamination, as well as an on-line control strategy to properly command the pumping and electrolysis operation.
However, there is no use that is recognized for hydrogen. The method is essentially an alternative to air sparging, with much higher efficiency in terms of oxygen transfer to the liquid phase (resulting in higher dissolved oxygen concentration and better diffusion), and with the additional benefit of hydroxyl radicals generation. The outcomes expected are an initiation of chemical oxidation and a stimulation of aerobic indigenous microbial populations (oxidative pathways).
In another prior art, U.S. Pat. No. 5,919,351, the patent uses in-situ electrolysis with two flat screened electrodes placed perpendicular to the water flow direction, and crossed by the water path. First electrode is negative (cathode) and generates hydrogen; second one is positive (anode) and generates oxygen. This creates two zones, anaerobic and aerobic, so that the treatment is sequential rather than simultaneous coupling due to the oxygen gradient across the biofilm. In addition, the distance between the electrodes is relatively large, in the meter range, so that high voltage has to be applied for enough current to be generated. In addition, water has to flow across the flat screened electrode, requiring large electrode areas, and a risk of electrode clogging, i.e. loss of electrolytical efficacy and permeability. Those characteristics jeopardize the cost-effectiveness of the system, further to the fact that this method is limited to shallow aquifers.
An integrated anaerobic and aerobic system for bioremediation of groundwater is described in our previous U.S. Pat. No. 5,599,451, the disclosure of which is incorporated herein by reference (Guiot 1997a). Although this system has been found to be quite useful for a variety of applications and compounds (Guiot 1997b, Tartakovsky et al. 2001), the low solubility of oxygen in water and low density of biomass granules cause certain problems.